Wiring new electric range

[Phil S.]

Our old range finally gave out.  The selector switch on the oven was snafu and wouldn't turn off the oven.  I talked with an appliance repair outfit about getting a new switch, but they recommended it was time for a new stove.  Heck, it was only 30+ years old, but daughter said that when the guy selling parts says buy a new one (which they don't sell), you should probably listen to them.  So we ordered one from HD and after a 12 week wait 
 it is sitting in the kitchen.

I figured this would be straight forward - pull the old one out, plug the new one in and start cooking, but then it got tricky.  The old stove had a 3 prong plug so we ordered a 3 prong cord with the new stove, but when I go to switch them out I find another wire, a 12 ga. with a white sheath fastened to the back of the old stove with a green screw.  I pull the cover off the 3 prong outlet and the mystery wire is fastened under one of the screws holding the outlet bracket to the wall.  There's a bare wire fastened under another screw through the bracket that follows the red, black and white cables into the wall (6/3 w/gnd, I think) to a 50 amp breaker.

So---is this really a half ummm baked 4 prong outlet? 
  I'm starting to feel it's time for somebody who knows what they're doing to take a look at this.

[TDKPE]

Got a picture?  I'm not clear on what you're describing.

If the existing receptacle is wired with three conductors, and has three slots for a three-prong plug, it's grandfathered and is ok to use as-is with a NEMA 10-50 plug (single-phase, 125/250V, 50A, 3-pole, ungrounded).  The frame of the oven needs to be bonded to the neutral in this case.  

If there is a four-slot NEMA 14-50 receptacle (single-phase, 125/250V, 50A, 4-pole, grounded), use a 4-conductor cord, with the frame NOT bonded to the neutral.  But do make sure all four poles are utilized, with a 4-wire feeder.

Edit: Rereading it a couple of times, it sounds like it is a half-baked separate ground, but you'd have to chase it back to the panel to be sure there really are four conductors all the way from the panel to the back of the stove. And if so, it should have a 14-50 plug and 4-conductor cord to a matching receptacle, with 4 conductors back to the panel, with no connection between the ground and the neutral other than in the main panel, or service equipment if that's not where the grounds are bonded to the neutrals.

And check the documentation for the ovens - make sure it's supposed to be on a 50A circuit. It's possible it should be on a smaller breaker, but that's easy enough (and cheap enough) to change out, with no need to downsize the conductors.

[Phil S.]

Got a picture?  I'm not clear on what you're describing.

If the existing receptacle is wired with three conductors, and has three slots for a three-prong plug, it's grandfathered and is ok to use as-is with a NEMA 10-50 plug (single-phase, 125/250V, 50A, 3-pole, ungrounded).  The frame of the oven needs to be bonded to the neutral in this case.  

If there is a four-slot NEMA 14-50 receptacle (single-phase, 125/250V, 50A, 4-pole, grounded), use a 4-conductor cord, with the frame NOT bonded to the neutral.  But do make sure all four poles are utilized, with a 4-wire feeder.

Edit:  Rereading it a couple of times, it sounds like it is a half-baked separate ground, but you'd have to chase it back to the panel to be sure there really are four conductors all the way from the panel to the back of the stove.  And if so, it should have a 14-50 plug and 4-conductor cord to a matching receptacle, with 4 conductors back to the panel, with no connection between the ground and the neutral other than in the main panel, or service equipment if that's not where the grounds are bonded to the neutrals.

And check the documentation for the ovens - make sure it's supposed to be on a 50A circuit.  It's possible it should be on a smaller breaker, but that's easy enough (and cheap enough) to change out, with no need to downsize the conductors.

Tom-

Thanks for your feedback.  Here's some pics.  First the wall connection-

The white wire on the left runs to the stove.  The ground wire from the cable is attached to the screw on the right.  There is no slot to accommodate the white wire.  The cover is just jimmied over it and tightened down.  I haven't backed out the screws holding the ground wires, but am assuming they are just round head wood screws holding the back of the outlet box to the board behind it.  There is continuity between the white wire going to the old stove and the bare wire going into the cable.

Next is the connection to the stove.  The 3 wire harness is attached with the white neutral attached to the center screw and the red and black hot leads attached to the left and right screws.  The neutral screw has a metal strap running to the grounding screw.  If a 4 wire harness is used that strap is severed with a tin snips and a piece of it removed.  The fourth wire (ground) is then attached to the green screw.

Documentation for the stove says it can used either a 40 or 50 amp breaker.

[EdL]

Just get rid of the extra wires and hook it up as the instructions say. Trace the added wire to the panel and remove it.

I've got a headache trying to comprehend what someone was thinking when they added the extra wire......let alone explain it.

I would inspect the wires going back to the panel and check the panel. If there was a ground issue, what they added isn't big enough.

Ed

[JTTHECLOCKMAN]

Man have not seen those outlets in quite some time. That is old.

[Phil S.]

I think this place was built in the 70's and early 80's.  We bought it in '89.  Originally it had plumbed gas lights, wood heat and a generator to run a washing machine.  The initial electrical system ran off the generator.  Eventually a marine cable was run out here and they got power from the utility, but didn't do much to upgrade the system.  Wife was home and heard the panel humming.  She got a wooden spoon and turned off all the breakers (60 amp panel) and called an electrician.  He came out and found the system wasn't grounded - at all!  We had a 200 amp panel installed and have been working on upgrading things as we find them.  The marine cable was replaced with an overland shielded cable when a fellow down the beach hauled it up on his anchor.  We got a road out here in 2006.  The neighborhood went together and had a new transformer installed this summer.  The previous owner was, ah, frugal and somewhat creative with the work he did.  This outlet with the accessory ground wire fits right in with other things here.

[blackhat]

The rough in for the stove wire was done with 3 conductor cable with ground and then an old style receptacle was put on it. To be "safe", an external wire was run from the stove to the box, grounding the stove. Best bet would be to buy a stove receptacle, box, 40 amp breaker and stove cord and install them onto the cable and stove.

[TDKPE]

The rough in for the stove wire was done with 3 conductor cable with ground and then an old style receptacle was put on it. To be "safe", an external wire was run from the stove to the box, grounding the stove. Best bet would be to buy a stove receptacle, box, 40 amp breaker and stove cord and install them onto the cable and stove.

OP:  That's what I would do, assuming that's actually a 4-conductor cable from the panel to the receptacle.  It looks like it from here, but you're in the best position to know.

Also, looking closely at the back of the stove, it looks like there is a bonding link between the frame and the neutral (black strap to the green screw), though I could be wrong.  If you use a 4-wire cord to the stove, be sure to remove that bonding strap, or you'll be passing neutral current through the grounding system, which is a no no.  The connections aren't designed for that, and the grounding conductor looks to be smaller than the current-carrying conductors (10 gauge copper equipment grounding conductor is good for up to a 60A circuit), plus it's uninsulated other than the sheath.

If sticking with the 3-wire cord, remove and toss that white ground (
) and make sure the bonding strap is actually bonding the frame to the neutral.

[Roly]

OP:  That's what I would do, assuming that's actually a 4-conductor cable from the panel to the receptacle.  It looks like it from here, but you're in the best position to know.

Also, looking closely at the back of the stove, it looks like there is a bonding link between the frame and the neutral (black strap to the green screw), though I could be wrong.  If you use a 4-wire cord to the stove, be sure to remove that bonding strap, or you'll be passing neutral current through the grounding system, which is a no no.  The connections aren't designed for that, and the grounding conductor looks to be smaller than the current-carrying conductors (10 gauge copper equipment grounding conductor is good for up to a 60A circuit), plus it's uninsulated other than the sheath.

If sticking with the 3-wire cord, remove and toss that white ground (
) and make sure the bonding strap is actually bonding the frame to the neutral.

Or is the white wire grounding something else ?    Seems like the receptacle was already grounded by the bare wire.  Check where the white wire goes,  as it is probably providing a ground to something.   What ever it is may need to be properly grounded.   Roly
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[Phil Thien]

The white wire is just under another screw on the box. The OP said it attached to the old range. It was just another chassis ground. Probably a good idea with a 30YO appliance but I'd be concerned that a modern appliance may do something extraordinary with the chassis.

I'd wire the new up exactly like the old one, just skip the extra white wire they have used for a ground. Just unscrew it and set it aside.